A pyrolysis-free Ni/Fe bimetallic electrocatalyst for overall water splitting

Catalysts capable of electrochemical overall water splitting in acidic, neutral, and alkaline solution are important materials. This work develops bifunctional catalysts with single atom active sites through a pyrolysis-free route. Starting with a conjugated framework containing Fe sites, the addition of Ni atoms is used to weaken the adsorption of electrochemically generated intermediates, thus leading to more optimized energy level sand enhanced catalytic performance. The pyrolysis-free synthesis also ensured the formation of well-defined active sites within the framework structure, providing ideal platforms to understand the catalytic processes. The as-prepared catalyst exhibits efficient catalytic capability for electrochemical water splitting in both acidic and alkaline electrolytes. At a current density of 10 mA cm−2, the overpotential for hydrogen evolution and oxygen evolution is 23/201 mV and 42/194 mV in 0.5 M H2SO4 and 1 M KOH, respectively. Our work not only develops a route towards efficient catalysts applicable across a wide range of pH values, it also provides a successful showcase of a model catalyst for in-depth mechanistic insight into electrochemical water splitting.

Zang and collaborators developed a bi-function catalyst with single atomic active sites through pyrolysisfree route, the as-prepared catalyst showed an out-standing catalytic capability as a bifunctional HER and OER pH-universal catalyst. This work reported an interesting catalytic performance for overall water splitting based on Fe/Ni active sites. According to the results, the overpotential for HER/OER were 23/201 mV and 42/194 mV in 0.5 M H2SO4 and 1 M KOH, respectively. In addition, the catalyst also exhibited considerable stability. The authors also performed the theoretical calculations in detail. Overall, these findings are impressive and well presented and organized. This work is also a successful showcase for new synthetic routes of single atomic catalysts. Thus, this work is acceptable for Nat. Commun. Here are some minor concerns which should be carefully addressed before the paper can be published: With regard to the synthesis of CPF-Fe/Ni material, CPF-Fe was synthesized by a pyrolysis-free route firstly. The authors should add more discussion for the possible special characteristic of such synthesized materials compared with the Fe single atoms based materials by pyrolysis route.
The reported CPF-Fe/Ni was subsequently prepared via a post-modification route, in which the ion exchange reaction was continuously triggered under a microwave reaction to generate the bimetallic conjugation of CPF-Fe/Ni. Compared with the catalysts prepared by one-pot method or in situ substitution by a heterometal ion, are there some unique features of CPF-Fe/Ni as a bifunctional catalysts for HER and OER?
The authors have performed lots of tests to study the stability of the catalysts and compared such as FTIR, TEM, HAADF STEM before/after the catalysis process to verify the stability. The authors should also include the comparison of XPS tests before/after the catalysis process to further confirm the stability of elemental state. Thus, XPS data tested under the same procedure after catalysis process should be added.
Some issues about the figures should revised. For example, the colors of hydrogen and oxygen are contrary in the note of supplementary Figure 19-21. The format should be uniform both in the manuscript and the supporting information, such as "SupplementaryFigure 2." should be "Supplementary Some grammar and format mistakes should be corrected. The authors are suggested to check the manuscript as well as the supporting information.
Reviewer #3 (Remarks to the Author): Comments This work developed the single-atom electrocatalyst with bimetallic sites, which was highly efficient for overall water splitting in wide-pH range. Impressively, this catalyst achieved demonstrate low overpotential with high stability for overall water splitting in both acidic and alkaline electrolytes. The authors have conducted substantial characterizations to verify their outcomes. The research has been well performed and the results are convincing. The pyrolysis-free synthesis for the preparation of highly efficient Fe and Ni based catalysts holds valuable novelty, which also is a good showcase for the catalytic process studies. Therefore, I would like to recommend the publication in Nature Communication. Following are some minor issues which could be addressed before final publication: 1. The authors have claimed that "the pyrolysis-free route was mild and ensure the well-defined structure and precise location of active centers, offering a new path for further explanation of the mechanism". Since such synthesis for single atomic catalysts is the important novelty of this work, the authors should provide more description or discussion for pyrolysis-free. 2. According to the results reported by the authors, it could be concluded that the presence of Ni was very important or even essential for the catalytic performance during the overall water splitting. Thus, is it possible to supply more study about the control of Ni species during the synthesis? For example, to explore whether the Ni content can be affected by the reaction conditions. 3. As the authored reported, Ni single atoms were introduced by a post-synthesized. During this process, it could be noted that KOH was used in the synthesis. What the role did KOH play, and how to avoid Ni(OH)2 was produced? 4. This work involved a large number of literature comparisons, which was very necessary. These comparisons need to be guaranteed under the same conditions. The authors are suggested to clearly mark the comparison conditions. 5. Given to the scientificity and the continuity of the data, the video of overall water splitting process is more intuitive and easy to accept. The authors are suggested to provide the videos of the water splitting. 6. Some typos and errors in grammar and format need to be corrected: On Page 3, Line 9, "have attracted numerous attentions" should be corrected for "have attracted numerous attention". On Page 4, Line 11, "various routes have been developed to improve the atomic utilization over the past decades." should be corrected for "various routes had been developed to improve the atomic utilization over the past decades". Similar mistake also found on Page 5, Line 7 and 8. The format of references should be unified. Such as in Ref 32 "Energy Environ. Sci., 15" should be "Energy Environ. Sci. 15". Meanwhile, in Ref 10 "CO2" should be "CO2".

Reviewer #1
The authors present an interesting work on a two-dimensional conjugated phthalocyanine framework (CPF) simultaneously contained single atomic Ni and Fe sites with improved activity towards overall water splitting in wide-pH range. It is noteworthy that the CPF-Fe/Ni exhibited excellent stability for OER in acidic media, which is always considered to require the use of noble metal catalysts such as Ir or Ru. The work would be of broad interest to the research community. I'd be happy to recommend acceptance of this study after the authors address the following points: Response: Thanks so much for the comments. Response: Many thanks for the advice. There are some other ways to solve the mentioned problem including the three-dimensional open nano-netcage electrocatalysts and it is very necessary to refer these studies in order for a better understanding. According to the reviewer's suggestion, we performed a more detailed literature research and discussion about the ways to improve atomic utilization. All the changes are highlighted in yellow background: "For example, derivatives of three-dimensional MOFs (Metal-Organic Frameworks) with open nanostructures can provide a frameworks with large area for catalysis. It has been developed that three-dimensional open nano-netcage derived electrocatalysts could effectively improve atomic utilization rate. Nowadays, many porous materials such as nanosheets with rough surface and expanding layers, 17 three-dimensional open nano-netcage electrocatalysts 24 and nanotube structures with well-defined inner channels and a large surface area 25 can effectively improve atomic utilization rate.  Am. Chem. Soc. 140, 2610-2618(2018. Figure 1f, "In the aberration corrected images it could be found that the coordinated Fe and Ni sites were exclusively dispersed in the single-atom format and uniformly anchored throughout the surface." Frankly, I can't see it clearly.

In the
Response: Many thanks for the Reviewer's suggestion. To this concern, we have uploaded a new figure with higher resolution in the revised version for more clearly observation. The changes are highlighted in yellow background: Response: Thanks for the professional suggestion. Considering the reviewer's concerns, we have investigated more literatures about XANES spectra of M-N4 configuration and discovered many similar results as our results. According to the suggestions and for a better understanding, we have added the corresponding references and the discussions. Typically, the pre-edge peak is due to a 1s-4pz shakedown transition characteristic for a square-planar configuration with high D4h symmetry. It should be noted that the pre-edge profile of CPF-Fe/Ni is similar to that of FePc and NiPc, but exhibits a slight difference in intensities, suggesting that the Fe/Ni center is coordinated with four N atoms by achieving a square-planar Fe/Ni-N4 molecular structure. (Nat.  (2020)). In order to make the image information clearer, we enlarged the local area (inserted images) and provided a more delated discussion about the XANES spectra in the revised manuscript. All the changes have been highlighted in yellow background as followed: Fig. 2 The synchrotron-based XANES and the extended X-ray absorption fine structure (EXAFS) spectra. (a) Normalized XANES and (b) radial structure functions (RSFs) of the Fe K-edge obtained by Fourier transformation k 3 -weighted EXAFS results, with the Fe foil, Fe2O3, Fe3O4, and FePc serving as references; (c) Normalized XANES and (d) radial structure functions (RSFs) of the Ni K-edge obtained by Fourier transformation k 3 -weighted EXAFS results, with the Ni foil, Ni(OH)2, NiO, and NiPc serving as references.
According to previous reports, the pre-edge peak could be attributed to a 1s-4pz shakedown transition characteristic for a square-planar configuration with high D4h symmetry. 32,33 In this work, it should be noted that the pre-edge profile of CPF-Fe/Ni is similar to that of iron phthalocyanine (FePc) and nickel phthalocyanine (NiPc) ( Fig.  2a and c), but exhibits a slight difference in intensities. These phenomenons suggested that the Fe/Ni center is coordinated with four N atoms by achieving a square-planar Fe/Ni-N4 molecular structure. 34

It's unclear if the electrochemical results have been compensated with solution
resistance in the manuscript, and I suggest the Tafel analysis use the compensated data. When using the tafel slopes to discuss the reaction kinetics, the author needs to select a region with a relatively small current density to ensure that it is in the kinetics region.
Response: Many thanks for the professional advice. As the reviewer mentioned, compensating data with solution resistance through iR-compensation is very important in the testing process and for the related analysis. It is one of the essential requirements for the electrochemical tests. In our work, all of the measurements and the as-obtained data were calibrated with iR-compensation including the Tafel analysis. When calculating the Tafel slope, the minimum current density region that meets kinetics region the required has been used. We are sorry that we did not clearly indicate in the test conditions. We emphasized the related parts in the revised manuscript and added the information in the revised experimental section. All the changes have been highlighted in yellow background as followed: All polarization curves were corrected for the iR compensation (the specific percentage of the correction is 100%).
5. In the "Electrochemical measurements" section," All electrode potentials reported herein were converted to the RHE scale using E(vs. RHE) = E(vs. SCE) + 0.197 V and E(vs. RHE) = E(vs. SCE) + 1.023 V for the measurements in acidic and alkaline media, respectively". To ensure the authenticity of electrochemical data, the zero point of RHE needs to be calibrated using the HER/HOR equilibrium potential using Pt/C catalyst in H2-saturated solutions.
Response: Thanks for the professional suggestions. As the reviewer mentioned, it is very important and essential to calibrate the zero point of RHE. During the tests, we have calibrated the RHE by the equilibrium potential of HER/HOR using Pt/C catalyst in H2-saturated solutions. We are very sorry that we missed the statement and made it unclearly. In case of any misunderstandings, we added this very important information in the revised manuscript. All the changes have been highlighted in yellow background as followed: …for the measurements in acidic and alkaline media, respectively. The zero point of RHE was determined by the equilibrium potential of HER/HOR using Pt/C as working electrode in H2 -saturated electrolyte.
6. The authors claim that "Moreover, the differential charge results revealed that the electron transferring occurred between the Fe in different layers, while there was no interaction between Fe and Ni (Fig. 5d)." In my opinion, the authors need to give further experimental evidence to explain the high activity of CPF-Fe/Ni.

Response:
Thanks so much for the professional advice. In case for the concern, we have added more discussion based on the experimental results to further explain the interaction between Fe and Ni, as well as the relate activity of CPF-Fe/Ni. The changes are highlighted in yellow background as followed: Moreover, the differential charge results revealed that the electron transferring occurred between the Fe in different layers, while there was no interaction between Fe and Ni (Fig. 5d). As revealed by the experimental results, the coordinated Fe and Ni sites were exclusively dispersed in the single-atom format and uniformly anchored throughout the surface in the HAADF STEM images (Figure 1f). Besides, the synchrotron-based XANES and the extended X-ray absorption fine structure (EXAFS) spectra also confirmed that the existence of metal-N coordination and the absence of metal-metal bonds. The absence of WT singles located around ~5.2 Å -1 that derived from Ni-metal bond and Fe-metal bond 39 further prove that no Ni-Fe bond exits in CPF-Fe/Ni. Thus, combine experimental and theoretical calculations, it could be concluded that there was no interaction between Fe and Ni. Meanwhile 7. Figure 3c and f -the chronopotentiometry curves -very stable. Please specify the corresponding reactions, HER or OER. Similar problems - Supplementary Figure 10 and 11, please specify the corresponding reactions.
Response: Many thanks for the careful advices. We have specified the discussion of the concerned parts and added more detailed description in case of any misleading. The changes have been highlighted in yellow background in the revised version as followed: "Then, a three-electrode cell configuration was employed with Ag/AgCl as the reference electrode. Both the working electrode and the counter electrode are drop-coated with carbon cloth of CPF-Fe/Ni. During the catalysis reaction, the cathode and anode produced hydrogen evolution reaction and oxygen evolution reaction, respectively. The current of the cathode was collected to evaluate the catalytic activity and stability of CPF-Fe/Ni as hydrogen evolution and oxygen evolution bifunctional catalysts."  Supplementary Figure 8. HER polarization curves with a speed of 10 mV s -1 in a wide pH.
Supplementary Figure 9. OER polarization curve with a speed of 10 mV s -1 in a wide pH.

With regard to the synthesis of CPF-Fe/Ni material, CPF-Fe was synthesized by a pyrolysis-free route firstly. The authors should add more discussion for the possible special characteristic of such synthesized materials compared with the Fe single atoms based materials by pyrolysis route.
Response: Many thanks for the suggestions. In the revised version, we have added a depth discussion according to the suggestion. The changes have been highlighted in yellow background as followed: Pyrolysis indeed has been very widely used in the past decades. Unfortunately, pyrolysis process is always carried out in specific atmosphere with high temperature, which are very energy-intensive. Meanwhile, the isolated metal atoms on the precursors are very unstable and tend to agglomerate during the pyrolysis process, not only reducing the efficient utilization of metal active sites but also making it difficult to realize the precise control. Thus, the development of non-carbonized synthesis strategies for the preparation of efficient electro-catalysts is challenging and vital. It could be expected that the pyrolysis-free route would lead to precisely synthetic control and a better maintained structure, holding the potentials to elucidate structure-function relationships for the guidance of optimum electro-catalysts.

The reported CPF-Fe/Ni was subsequently prepared via a post-modification route, in which the ion exchange reaction was continuously triggered under a microwave reaction to generate the bimetallic conjugation of CPF-Fe/Ni. Compared with the catalysts prepared by one-pot method or in situ substitution by a heterometal ion, are there some unique features of CPF-Fe/Ni as a bifunctional catalysts for HER and OER?
Response: Thanks for the comments. In this work, the bimetallic configuration of CPF-Fe/Ni was synthesized via a post-modification under a microwave reaction.
Compared with commonly used pyrolysis or one-pot solvothermal method, the synthesis route is more mild, easy to perform and conducive to the controllability of the synthesis process. This method could not only effectively reduce the formation of agglomeration, but also maintain the integrity of the structure to the greatest extent since the drastic treatments such as etching process were avoided. Moreover, the reactions are very simple, which hold the promising potentials for large-scale production. We have added the comparison in the revised version and marked in yellow background as followed: … simultaneously contained single atomic Ni/N/C and Fe/N/C (termed as CPF-Fe/Ni). The mild synthetic route not only ensured the effectively reduce the formation of agglomeration, but also maintain the integrity of the structure. Response: Many thanks for the professional suggestions. According to the suggestion, we have collected the XPS results after the catalysis process both in acid and alkaline conditions, which showed a highly consistent with other measurement and further confirmed the stability of elemental state. The corresponding results have been added in the Supporting Information as Supplementary Figure 18 and 19. The changes of the relative parts in the manuscript are as followed. All the changes are highlighted in yellow background.
High-resolution N1s and Fe2p spectra of CPF-Fe/Ni after electrocatalysis testing in 0.5 M H2SO4 and 1 M KOH were collected (Supplementary Fig. 18 and 19), which also showed a highly consistent with other measurement and further confirmed the stability of elemental state. Response: Many thanks for the careful advice. We have corrected the mentioned concerns in the revised version in case of any misleading. The colors of hydrogen and oxygen in the note of supplementary Figure 21-23 have been corrected as "Hydrogen (green) and oxygen (red) generated at 0, 1. 12, 2,23, 3.43, 4.58, 5.77, 7, 8.25, 9.5, and 10.83 min in 0.5 M H2SO4, respectively" and "Hydrogen (green) and oxygen (red) generated at 0, 3.55, 6.1, 8.45, 10.76, 13.02, 15.42, 17.67, 20.03, and 22.38 min in 1 M KOH, respectively." The format in the Supplementary Text have been united. All the changes are highlighted in yellow background as followed:

Supplementary
Supplementary Figure 21. Hydrogen (green) and oxygen (red) generated at 0, 1. 12, 2,23, 3.43, 4.58, 5.77, 7, 8.25, 9.5 Response: Thanks so much for the advice. We have corrected the issues as " Supplementary Figure 6. Tafel slopes for 20% Pt-C, CPF-Fe, CPF-Ni and CPF-Fe/Ni as HER catalyst in 1 M KOH". We also have checked through the whole manuscript and the supporting information in case of such mistakes. The changes are highlighted in yellow background.
Supplementary Figure 6. Tafel slopes for 20% Pt-C, CPF-Fe, CPF-Ni and CPF-Fe/Ni as HER catalyst in 1 M KOH

Reviewer #3
This work developed the single-atom electrocatalyst with bimetallic sites, which was highly efficient for overall water splitting in wide-pH range. Impressively, this catalyst achieved demonstrate low overpotential with high stability for overall water splitting in both acidic and alkaline electrolytes. The authors have conducted substantial characterizations to verify their outcomes. The research has been well performed and the results are convincing. The pyrolysis-free synthesis for the preparation of highly efficient Fe and Ni based catalysts holds valuable novelty, which also is a good showcase for the catalytic process studies. Therefore, I would like to recommend the publication in Nature Communication. Following are some minor issues which could be addressed before final publication: Response: Thanks so much for the comments.
1. The authors have claimed that "the pyrolysis-free route was mild and ensure the well-defined structure and precise location of active centers, offering a new path for further explanation of the mechanism". Since such synthesis for single atomic catalysts is the important novelty of this work, the authors should provide more description or discussion for pyrolysis-free.
Response: Many thanks for the professional suggestions. In order for the deep understanding, we have added more description and discussion about pyrolysis-free in the revised manuscript. The changes have been highlighted in yellow background as followed: Pyrolysis indeed has been very widely used in the past decades. Unfortunately, pyrolysis process are always carried out in specific atmosphere with high temperature, which are very energy-intensive. Meanwhile, the isolated metal atoms on the precursors are very unstable and tend to agglomerate during the pyrolysis process, not only reducing the efficient utilization of metal active sites but also making it difficult to realize the precise control. Thus, the development of non-carbonized synthesis strategies for the preparation of efficient electro-catalysts is challenging and vital. It could be expected that the pyrolysis-free route would lead to precisely synthetic control and a better maintained structure, holding the potentials to elucidate structure-function relationships for the guidance of optimum electro-catalysts. Response: Many thanks for the professional comments. According to reported work, we found that CPF-Fe support can be well dispersed under alkaline conditions. KOH could provide a good solvent for the reaction. Thus, the addition of KOH could increase the interaction between Ni ion and CPF-Fe and promote the formation of bimetallic single-atom catalysts. In the subsequent process, a certain amount of HCl was used to remove excess impurities, which further avoid the production of Ni(OH)2. The detailed steps can be found in "Synthesis of CPF-Fe/Ni" of the manuscript. We have also added the description. The changes are marked in yellow background as followed: It has been reported that the CPF-Fe support could be well dispersed under alkaline conditions. Thus, KOH was used as the solvent for the reaction to promote the formation of bimetallic single-atom catalysts.
4. This work involved a large number of literature comparisons, which was very necessary. These comparisons need to be guaranteed under the same conditions. The authors are suggested to clearly mark the comparison conditions.
Response: Many thanks for the professional suggestions. We have uniformed the literature comparisons to guaranteed they are all under the same conditions (10 mA cm -2 ). We also added the detailed conditions. All the changes have been marked in Table S1 and highlighted in yellow background as followed: 5. Given to the scientificity and the continuity of the data, the video of overall water splitting process is more intuitive and easy to accept. The authors are suggested to provide the videos of the water splitting.
Response: Many thanks for the professional suggestions. The videos of overall water splitting process in 0.5 M H2SO4 and 1M KOH have been provided in the revised version as supplied information and referred in the related part of the manuscript. The changes are all marked in yellow background.
…which was performed in a sealed H-type cell wherein CPF-Fe/Ni was acted as both cathodic and anodic electrodes (Supporting Information Fig.23 and 25, Video 1 and 2).
In the revised supporting information: Caption of video 1. The video of overall water splitting process in 0.5 M H2SO4.
Caption of video 2. The video of overall water splitting process in 1M KOH.
6. Some typos and errors in grammar and format need to be corrected: On Page 3, Line 9, "have attracted numerous attentions" should be corrected for "have attracted numerous attention". On Page 4, Line 11, "various routes have been developed to improve the atomic utilization over the past decades." should be corrected for "various routes had been developed to improve the atomic utilization over the past decades". Similar mistake also found on Page 5, Line 7 and 8. The format of references should be unified. Such as in Ref 32 "Energy Environ. Sci.,15" should be "Energy Environ. Sci. 15". Meanwhile,in Ref 10 "CO2" should be "CO2".
Response: Many thanks for the careful suggestions. We have corrected the mentioned typos, grammar issues and formats of references. We also have checked through the whole manuscript and the supporting information in case of such mistakes. The changes are highlighted in yellow background.
"have attracted numerous attentions" have been corrected for "have attracted numerous attention" "various routes have been developed to improve the atomic utilization over the past decades." have been corrected for "various routes had been developed to improve the atomic utilization over the past decades".

REVIEWERS' COMMENTS
Reviewer #1 (Remarks to the Author): The revised manuscript can be accepted now.
Reviewer #2 (Remarks to the Author): After the revision, this work could be accpeted.

Comments
This work developed the single-atom electrocatalyst with bimetallic sites, which was highly efficient for overall water splitting in wide-pH range. Impressively, this catalyst achieved demonstrate low overpotential with high stability for overall water splitting in both acidic and alkaline electrolytes. The authors have conducted substantial characterizations to verify their outcomes. The research has been well performed and the results are convincing. The pyrolysis-free synthesis for the preparation of highly efficient Fe and Ni based catalysts holds valuable novelty, which also is a good showcase for the catalytic process studies. All comments by the reviewers have been adressed in detail, thus the manuscript can be accepted for publication.